High-Accuracy, Quick-Change, Robot Factory Interface

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High-Accuracy, Quick-Change, Robot Factory
Interface
John Hart (ajhart_at_mit.edu)Prof. Alexander
Slocum, AdvisorMIT Precision Engineering
Research Group
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Project Goals
Design, test, and demonstrate production
feasibility of a modular robot baseplate with
kinematic couplings as locators

• A repeatable, rapidly exchangeable interface
  between the foot (three balls/contactors) and
  floor plate (three grooves/targets)
• Calibrate robots at ABB to a master baseplate
• Install production baseplates at the customer
  site and calibrated the kinematic couplings
  directly to in-cell tooling
• Install robot according to refined mounting
  process with gradual, patterned preload to
  mounting bolts
• TCP-to-tooling relationship is a deterministic
  frame transformation
• Base calibration data handling is merged with ABB
  software, enabling 0.1 mm TCP error contribution
  from repeatability and exchangeability error of
  kinematic couplings

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Prototype Coupling Designs
Design 3-point kinematic coupling mounts for the
6400R foot

• Canoe Ball
• Six point contacts
• 0.5m radius ball surface
• 20 mm diameter elastic Hertzian contact

• Three-Pin
• Three line three surface contacts
• In-plane preload overcomes friction to
  deterministically seat pins
• Vertical bolt preload engages horizontal contact
  surfaces

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Prototype Coupling Designs

• Groove/Cylinder
• Twelve line contacts
• Aluminum cylinders
• Apply bolt preload (elastic deflection of
  cylinders) for dynamic stability

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Prototype Base Mounting
Tests at ABB Robotics Vasteras, July/August 2001

• Static (bolted) and dynamic (5-point path)
  repeatability of canoe ball and three-pin
  interfaces
• Static (manipulator rest only) repeatability of
  groove/cylinder interface
• Test both basic (air wrench) and refined (torque
  wrench, greased bolts) mounting processes
• Measure tool point motion using Leica LTD500
  Laser Tracker
• Repeatability of robot path measurement system
  approximately 20 microns

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Repeatability Performance

• Canoe balls vs. BMW base 83 reduction
• Three-pin vs. BMW base 85 reduction
• Cylinders vs. BMW base 92 reduction
• Refined mounting vs. basic mounting 50-70
  reduction
• 8-bolt blue pallet repeatability (not shown)
  1.63 mm

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Interchangeability Error Model
Consider stackup of errors in coupling
manufacturing, mounting plate manufacturing, and
coupling-to-plate assembly

• For example in z-direction of a ball mount,
  tolerances
• Sphere radius dRsph
• Contact point to bottom plane dhR
• Measurement feature height dhmeas
• Protrusion height dhprot

Each dimension is perturbed by generating a
random variate, e.g. for mounting hole placement
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Interchangeability Solution Method
Linear system of 24 constraint equations between
the balls and grooves accounts for both
positional and angular misalignment

• Contact sphere centers must be at minimum
  (normal) distance between the groove flats, e.g.
• By geometry, the combined error motion of contact
  spheres is known with respect to the error motion
  of their mounting plate. For small angles, e.g.
• Solve linear system and place six error
  parameters in HTM

q1, b1 initial, final center positions N1
groove normal R1 sphere radius.
(qS,1, qS,1, qS,1) initial center positions
(xS,1, yS,1, zS,1) final center positions.
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Interchangeability Results
Simulate interchangeablity error from
manufacturing variation

• Calibrate interfaces by measuring contacts and
  calculating interface error transformation
• Model direct measurement of pins contacts, and
  offset measurement of canoe balls
• Exchangeability is error between calculated and
  true interface transformation, given chosen level
  of calibration and manufacturing tolerances (low,
  med, high)
• 250-trial Monte Carlo simulation in MATLAB at
  each calibration level

Three-pin interchangeability
0 no interface calibration 3 full (x,y,z) of
pins and contact surfaces
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Total Mechanical Accuracy
Quick-Change Accuracy Repeatability
Exchangeability
(measured)
(simulated)
Canoe balls Three-pin Groove/cylinder
0.22 mm 0.06 0.16 0.12 mm 0.07
0.05 - 0.06 (Incomplete)

• Interface calibration decouples accuracy from
  manufacturing tolerances of mounting plates and
  couplings (if direct measurement of contacts)
• Results show repeatability is highly f(mounting
  process) this may present a performance limit
  for factory mountings interface should be
  micron-repeatable under perfect conditions
• Totally, a near-deterministic prediction of robot
  interface accuracy

driven by error of offset position
measurement static only
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Recommended Next Steps

• Test groove/cylinder interface with preload
  motion
• Test traditional quasi-kinematic couplings
• Evaluate long-term dynamic performance
• Production three-pin adaptation to BMW base
• Canoe ball 4-point mounting for Voyager?
• Build kinematic coupling Expert System
  combine test results, simulation results, etc.
  into design tool that gives minimum cost
  recommendation as f(accuracy requirement)